Method and apparatus for encoding/decoding images

ABSTRACT

Disclosed are a method and apparatus for encoding/decoding images. The image-decoding method comprises the steps of: receiving a bit stream including information regarding an NAL unit type; and checking whether or not the NAL unit in the bit stream is a reference picture based on said information regarding an NAL unit type and decoding the NAL unit. The information regarding an NAL unit type indicates whether the NAL unit is a reference reading picture or not a reference reading picture.

This application is a 35 USC §371 National Stage entry of InternationalApplication No. PCT/KR2013/008303 filed on Sep. 13, 2013, which claimspriority to U.S. Provisional Application No. 61/770,335, filed on Sep.13, 2012, all of which are incorporated by reference in their entiretyfor all purposes as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a video compression technique, and moreparticularly, to a method and apparatus for decoding image informationin a bitstream.

BACKGROUND ART

Recently, there is a growing demand on high-resolution, high-qualityimages in various application fields. With the increase in theresolution and quality of the image, an amount of information for theimage is also increased.

The increase in the information amount results in the introduction of adevice having various capabilities and a network having variousenvironments. With the introduction of the device of the variouscapabilities and the network having the various environments, the samecontent can be used with a variety of quality.

More specifically, as a user equipment can support an image of a varietyof quality and an implemented network environment is diversified, animage of normal quality is used in a certain environment, whereas animage of higher quality can be used in another environment.

For example, a consumer who purchases a video content in a portableterminal can watch the same video content with a larger screen and ahigher resolution by using a large-screen display at home.

A broadcasting service with a high definition (HD) resolution hasrecently been provided, and thus many users are accustomed tohigh-definition, high-quality images. Further, in addition to HDTV,service providers and users are paying attention to ultra highdefinition (UHD) having a resolution four times higher than the HDTV.

Therefore, in order to provide an image service requested by a user invarious environments according to quality in various manners, it isnecessary to provide a scalability to image quality (e.g., image picturequality, image resolution, image size, video frame rate, etc.) on thebasis of a high-efficient encoding/decoding method performed onlarge-capacity video. In addition, there is a need to discuss variousimage processing methods accompanied by such a scalability.

SUMMARY OF INVENTION Technical Problem

The present invention provides an image encoding/decoding method capableof improving an encoding/decoding efficiency.

The present invention also provides a bitstream extracting method andapparatus capable of improving an encoding/decoding efficiency.

The present invention also provides a network abstraction layer (NAL)unit type capable of improving an encoding/decoding efficiency.

Technical Solution

According to an aspect of the present invention, there is provided animage decoding method including: receiving a bitstream includinginformation on a network abstraction layer (NAL) unit type; and decodingan NAL unit by confirming whether the NAL unit in the bitstream is areference picture on the basis of the information on the NAL unit type,wherein the information on the NAL unit type is information indicatingwhether the NAL unit is a referenced leading picture or a non-referencedleading picture.

According to another aspect of the present invention, there is providean image decoding apparatus including an entropy decoder for receiving abitstream including information on an NAL unit type and for performingentropy decoding on an NAL unit by confirming whether the NAL unit inthe bitstream is a reference picture on the basis of the information onthe NAL unit type, wherein the information on the NAL unit type isinformation indicating whether the NAL unit is a referenced leadingpicture or a non-referenced leading picture.

According to another aspect of the present invention, there is providedan image encoding method including: generating a residual signal for acurrent picture by performing an inter prediction based on the currentpicture; and transmitting a bitstream including an NAL unit generatedbased on the residual signal for the current picture and information onthe NAL unit, wherein the information on the NAL unit includesinformation on an NAL unit type determined according to whether the NALunit is a referenced leading picture or whether the NAL unit is anon-referenced leading picture.

According to another aspect of the present invention, there is providedan image encoding apparatus including: a predictor for generating aresidual signal for a current picture by performing an inter predictionbased on the current picture; and an entropy encoder for outputting abitstream by performing entropy encoding on an NAL unit generated basedon the residual signal for the current picture and information on theNAL unit, wherein the information on the NAL unit includes informationon an NAL unit type determined according to whether the NAL unit is areferenced leading picture or whether the NAL unit is a non-referencedleading picture.

Advantageous Effects

Since a network abstraction layer (NAL) unit type is defined to providewhether an NAL unit is a reference picture referenced by a differentpicture or a non-reference picture not referenced by the differentpicture, the NAL unit can be effectively extracted from a bitstream. Inaddition, since whether the NAL unit is the non-reference picture iscorrectly derived, the NAL unit can be removed from the bitstreamwithout having an effect on a decoding process.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a video encoding apparatus according to anembodiment of the present invention.

FIG. 2 is a block diagram of a video decoding apparatus according to anembodiment of the present invention.

FIG. 3 shows a layered structure for a coded image processed in adecoding apparatus.

FIG. 4 shows a temporal layer structure for network abstraction layer(NAL) units in a bitstream supporting a temporal scalability.

FIG. 5 shows a temporal layer structure for NAL units in a bitstreamsupporting a temporal scalability to which the present invention isapplicable.

FIG. 6 is a diagram for explaining a randomly accessible picture.

FIG. 7 is a diagram for explaining an instantaneous decoding refresh(IDR) picture.

FIG. 8 is a diagram for explaining a clean random access (CRA) picture.

FIG. 9 shows a temporal layer structure for NAL units including aleading picture in a bitstream supporting a temporal scalability.

FIG. 10 is a diagram for explaining an operation of removing NAL unitsincluding a leading picture from a bitstream according to an embodimentof the present invention.

FIG. 11 is a flowchart showing an encoding method of image informationaccording to an embodiment of the present invention.

FIG. 12 is a flowchart showing a decoding method of image informationaccording to an embodiment of the present invention.

MODE FOR INVENTION

Since the present invention may have various modifications and diverseembodiments, only specific embodiments are exemplarily illustrated inthe drawings and will be described in detail. However, the presentinvention should not be construed as being limited to the specificembodiments set forth herein. The terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting of the invention. As used herein, the singular forms areintended to include the plural forms as well, unless the context clearlyindicates otherwise. In the present application, it is to be understoodthat the terms such as “including” or “having”, etc., are intended toindicate the existence of the features, numbers, operations, actions,components, parts, or combinations thereof disclosed in thespecification, and are not intended to preclude the possibility that oneor more other features, numbers, operations, actions, components, parts,or combinations thereof may exist or may be added.

Meanwhile, respective constructions in the drawings described in thepresent invention are illustrated independently for convenience ofexplanation regarding different particular functions in an imageencoding apparatus/decoding apparatus, and it does not imply that therespective constructions are implemented with separate hardware entitiesor separate software entities. For example, among the respectiveconstructions, two or more constructions may be combined into oneconstruction, and one construction may be divided into a plurality ofconstructions. Embodiments in which the respective constructions areintegrated and/or separated are also included in the scope of thepresent invention as long as not departing from the spirit of theinvention.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

FIG. 1 is a block diagram of a video encoding apparatus according to anembodiment of the present invention. A video encoding/decoding method orapparatus may be implemented through an extension of a typical videoencoding/decoding method which does not provide a scalability. The blockdiagram of FIG. 1 shows an embodiment of a video encoding apparatuswhich can be a basic scalable video encoding apparatus.

Referring to FIG. 1, an encoding apparatus 100 includes a picturedivider 105, a predictor 110, a transformer 115, a quantizer 120, are-arranger 125, an entropy encoder 130, a dequantizer 135, an inversetransformer 140, a filter 145, and a memory 150.

The picture divider 105 may divide an input picture on the basis of atleast one processing unit. In this case, the processing unit may be aprediction unit (PU), a transform unit (TU), or a coding unit (CU).

Processing unit blocks which are divided in the picture divider 105 mayhave a quad-tree structure.

The predictor 110, as described below, includes an inter predictor whichperforms an inter prediction and an intra predictor which performs anintra prediction. The predictor 110 generates a prediction block byperforming a prediction for a processing unit of a picture in thepicture divider 105. The processing unit of the picture in the predictor110 may be a CU, a TU, or a PU. In addition, the predictor 110 maydetermine whether a prediction performed for a corresponding processingunit is an inter prediction or an intra prediction, and may determine aspecific content (e.g., a prediction mode, etc.) of each predictionmethod. In this case, the processing unit for performing a predictionmay differ from the processing unit for determining the specificcontent. For example, a prediction method, a prediction mode, etc., maybe determined in a unit of PU, and the prediction may be performed in aunit of TU.

The inter prediction may be used to generate a prediction block byperforming a prediction on the basis of information of at least onepicture between a previous picture and/or a next picture of a currentpicture. In addition, the intra prediction may be used to generate aprediction block by performing a prediction on the basis of pixelinformation in the current picture.

As a method of the inter prediction, a skip mode, a merge mode, a motionvector prediction (MVP), etc., may be used. In the inter prediction,regarding the PU, a reference picture may be selected and a referenceblock corresponding to the PU may be selected. The reference block maybe selected in an integer pixel unit. Subsequently, a prediction blockis generated such that a residual signal with respect to the current PUis minimized and a size of a motion vector is also minimized.

The prediction block may be generated in an integer sample unit, and maybe generated in a pixel unit smaller than an integer unit, such as a ½pixel unit or a ¼ pixel unit. In this case, the motion vector may alsobe expressed in a unit smaller than an integer pixel.

Information of an index of a reference picture selected through theinter prediction, a motion vector (e.g., motion vector predictor), aresidual signal, etc., is subjected to entropy encoding and is thendelivered to the decoding apparatus. When the skip mode is applied,since a prediction block can be a reconstructed block, a residual maynot be generated, transformed, quantized, and transmitted.

When the intra prediction is performed, a prediction mode may bedetermined in a unit of PU, and thus a prediction may be performed inthe unit of PU. In addition, the prediction mode may be determined inthe unit of PU, and the intra prediction may be performed in a unit ofTU.

In the intra prediction, the prediction mode may have 33 directionalprediction modes and at least two non-directional modes. Thenon-directional mode may include a DC prediction mode and a planar mode.

In the intra prediction, a filter may be applied to a reference sampleand thereafter a prediction block may be generated. In this case,whether to apply the filter to the reference sample may be determinedaccording to an inter prediction mode and/or a size of a current block.

The PU may be a block having various sizes/shapes. For example, in caseof the inter prediction, the PU may be a 2N×2N block, a 2N×N block, aN×2N block, a N×N block, or the like (where N is an integer). In case ofthe intra prediction, the PU may be a 2N×2N block, a N×N block, or thelike (where N is an integer). In this case, it may be configured suchthat the PU having a size of the N×N block is applied only to a specificoccasion. For example, it may be configured such that the PU having asize of the N×N block is used only for a minimum-sized CU or is usedonly for the intra prediction. In addition to the aforementioned sizedPU, the PU may be further defined and used such as a N×mN block, a mN×Nblock, a 2N×mN block, or a mN×2N block (m<1).

A residual value (i.e., residual block or residual signal) between agenerated prediction block and an original block is input to thetransformer 115. In addition, prediction mode information used for theprediction, motion vector information, etc., may be coded in the entropyencoder 130 together with the residual value and may be delivered to adecoding apparatus.

The transformer 115 transforms the residual block in a unit of atransform block, and generates a transform coefficient.

The transform block is a rectangular block of samples, and is a block towhich the same transformation is applied. The transform block may be aTU, and may have a quad tree structure.

The transformer 115 may perform a transformation according to aprediction mode and a block size which are applied to the residualblock.

For example, if the intra prediction is applied to the residual blockand the block is a 4×4 residual array, the residual block may betransformed by using discrete sine transform (DST), and otherwise, theresidual block may be transformed by using discrete cosine transform(DCT).

The transformer 115 may perform a transformation to generate a transformblock of transformation coefficients.

The quantizer 120 may generate a quantization coefficient by quantizingresidual values, i.e., transformation coefficients, transformed in thetransformer 115. A value calculated by the quantizer 120 may be providedto the dequantizer 135 and the re-arranger 125.

The re-arranger 125 re-arranges the quantized transformationcoefficients provided from the quantizer 120. The re-arranging of thequantization coefficient may increase coding efficiency in the entropyencoder 130.

The re-arranger 125 may re-arrange quantization coefficients having aform of a 2-dimensional block into a format of a 1-dimensional vector byusing a coefficient scanning method.

The entropy encoder 130 may perform entropy encoding with respect to thequantization coefficients re-arranged by the re-arranger 125. Theentropy encoding may use Exponential Golomb, CAVLC (Context-AdaptiveVariable Length Coding), and/or CABAC (Context-Adaptive BinaryArithmetic Coding). The entropy encoder 130 may encode a variety ofinformation such as quantized transformation coefficient information andblock type information of a CU delivered from the re-arranger 125 andthe predictor 110, prediction mode information, division unitinformation, PU information and transmission unit information, motionvector information, reference picture information, interpolationinformation of a block, filtering information, etc.

In addition, the entropy encoder 130 may optionally add a specificchange in a parameter set or syntax to be transmitted.

The dequantizer 135 dequantizes values quantized in the quantizer 120(i.e., quantized transformation coefficients). The inverse transformer140 may inverse-transform values dequantized in the dequantizer 135.

Residual values generated in the dequantizer 135 and the inversetransformer 140 may be combined with a prediction block predicted in thepredictor 110, thereby generating a reconstructed block.

It is described in FIG. 1 that a residual block and a prediction blockare added by using an adder to generate a reconstructed block. In thiscase, the adder may be regarded as an additional unit (i.e., areconstructed block generator) for generating the reconstructed block.

The filter 145 may apply a de-blocking filter, an adaptive loop filter(ALF), and a sample adaptive offset (SAO) to a reconstructed picture.

The deblocking filter may remove block distortion which occurs at aboundary between blocks in the reconstructed picture. The ALF mayperform filtering on the basis of a value used to compare an originalimage with an image reconstructed after filtering a block through thede-blocking filter. The ALF may be performed only when high-efficiencyis applied. Regarding a residual block to which the de-blocking filteris applied, the SAO reconstructs an offset difference with respect to anoriginal image in a unit of pixel, and is applied in a form of a bandoffset, an edge offset, etc.

Meanwhile, regarding the reconstructed block used in the intraprediction, the filter 145 may not apply the filtering.

The memory 150 may store the reconstructed block or picture calculatedby using the filter 145. The reconstructed block or picture stored inthe memory 150 may be provided to the predictor 110 for performing theinter prediction.

FIG. 2 is a block diagram of a video decoding apparatus according to anembodiment of the present invention. As described in detail in FIG. 1,the video encoding/decoding method or apparatus may be implementedthrough an extension of a typical video encoding/decoding method whichdoes not provide a scalability. The block diagram of FIG. 2 shows anembodiment of a video decoding apparatus which can be a basic scalablevideo decoding apparatus.

Referring to FIG. 2, a video decoding apparatus 200 includes an entropydecoder 210, a re-arranger 215, a dequantizer 220, an inversetransformer 225, a predictor 230, a filter 235, and a memory 240.

When an image bitstream is input in the video encoding apparatus, theinput bitstream may be decoded according to a procedure by which imageinformation is processed in the video encoding apparatus.

For example, when a variable length coding (VLC) is used to performentropy encoding in the video encoding apparatus, the entropy decoder210 may also be implemented with the same VLC table as that used in theencoding apparatus to perform entropy decoding. In addition, when CABACis used to perform entropy encoding in the video encoding apparatus, theentropy decoder 210 may perform entropy decoding by using the CABAC inaccordance therewith.

Among information decoded in the entropy decoder 210, information forgenerating a prediction block is provided to the predictor 230, and aresidual value for which entropy decoding is performed in the entropydecoder 210, that is, a quantized transformation coefficient, may beinput to the re-arranger 215.

The re-arranger 215 may re-arrange the bitstream subjected to theentropy decoding in the entropy decoder 210, i.e., the quantizedtransformation coefficients, according to a re-arranging method used inthe encoding apparatus.

The re-arranger 215 may perform the re-arranging by reconstructingcoefficients expressed in a 1-dimensional vector format intocoefficients of a 2-dimensional block form. The re-arranger 215 mayperform scanning on a coefficient on the basis of a prediction mode anda transformation block size which are applied to a current block (i.e.,transformed block) to generate an array of coefficients (i.e., quantizedtransformation coefficients) having a format of a 2-dimensional block.

The dequantizer 220 may perform dequantization on the basis of aquantization parameter provided from the encoding apparatus and acoefficient value of a re-arranged block.

According to a result of quantization performed by the video encodingapparatus, the inverse transformer 225 may perform inverse DCT and/orinverse DST with respect to DCT and DST performed by the transformer ofthe encoding apparatus.

The inverse transformation may be performed on the basis of atransmission unit or image division unit determined in the encodingapparatus. The transformer of the encoding apparatus may selectivelyperform the DCT and/or the DST according to a plurality of pieces ofinformation such as a prediction method, a current block size, and/or aprediction direction, etc. The inverse transformer 225 of the decodingapparatus may perform inverse transformation on the basis of informationon transformation performed in the transformer of the encodingapparatus.

The predictor 230 may generate a prediction block on the basis ofprediction block generation-related information provided from theentropy decoder 210 and previously decoded block and/or pictureinformation provided from the memory 240.

If a prediction mode for a current PU is an intra prediction mode, anintra prediction for generating a prediction block may be performed onthe basis of pixel information in a current picture.

If the prediction mode for the current PU is an inter prediction mode,an inter prediction may be performed on the current PU on the basis ofinformation included in at least one picture between a previous pictureand a next picture of the current picture. In this case, informationregarding motion information required for the inter prediction of thecurrent PU provided by the video encoding apparatus, e.g., informationregarding a motion vector, a reference picture index, etc., may be usedto confirm a skip flag, a merge flag, etc., received from the encodingapparatus and may be derived in accordance therewith.

A reconstructed block may be generated by using a prediction blockgenerated from the predictor 230 and a residual block provided from theinverse transformer 225. It is described in FIG. 2 that a predictionblock and a residual block are added by using an adder to generate areconstructed block. In this case, the adder may be regarded as anadditional unit (i.e., a reconstructed block generator) for generatingthe reconstructed block.

When the skip mode is applied, a residual is not transmitted, and aprediction block may be regarded as a reconstructed block.

The reconstructed block and/or picture may be provided to the filter235. The filter 235 may apply a deblocking filter, a sample adaptiveoffset (SAO), and/or an adaptive loop filter (ALF), etc., to thereconstructed block and/or picture.

The memory 240 may store the reconstructed picture or block so as to beused as a reference picture or a reference block, or may provide thereconstructed picture to an output element.

Among the entropy decoder 210, re-arranger 215, dequantizer 220, inversetransformer 225, predictor 230, filter 235, and memory 240 included inthe decoding apparatus 200, constitutional elements directly related todecoding of an image, for example, the entropy decoder 210, there-arranger 215, the dequantizer 220, the inverse transformer 225, thepredictor 230, the filter 235, etc., may be expressed as a decoder or adecoding unit by distinguishing from other constitutional elements.

In addition, the decoding apparatus 200 may further include a parser(not shown) for parsing information related to an encoded image includedin the bitstream. The parser may include the entropy decoder 210, andmay be included in the entropy decoder 210. The parser may beimplemented with one constitutional element of the decoder.

FIG. 3 shows a layered structure for a coded image processed in adecoding apparatus.

The coded image is divided into a video coding layer (VCL) for handlinga process of image decoding or the process itself, a sub-ordinate systemfor transmitting and storing coded information, and a networkabstraction layer (NAL) which exists between the VCL and thesub-ordinate system and which manages a network adaptation function.

In the VCL, VCL data including compressed image data (i.e., slice data)may be generated, or a parameter set including information such as apicture parameter set (PPS), a sequence parameter set (SPS), a videoparameter set (VPS), etc., or a supplemental enhancement information(SEI) message additionally required for an image decoding process may begenerated.

In the NAL, an NAL unit may be generated by appending header information(i.e., an NAL unit header) to a raw byte sequence payload (RBSP)generated in the VCL. In this case, the RBSP implies slice data,parameter set, SEI message, etc., generated in the VCL. The NAL unitheader may include NAL unit type information specified according to theRBSP data included in the NAL unit.

As shown in FIG. 3, the NAL unit may be divided into a VCL NAL unit anda non-VCL NAL unit according to the RBSP generated in the VCL. The VCLNAL unit implies an NAL unit including image information (i.e., slicedata). The non-VCL NAL unit implies an NAL unit including informationrequired for image decoding (i.e., a parameter set or an SEI message).

The aforementioned VCL NAL unit and non-VCL NAL unit may be transmittedthrough a network by appending header information according to a dataprotocol of the sub-ordinate system. For example, the NAL unit may betransformed in a data format of a specific protocol such as H.264/AVCfile format, RTP (Real-time Transport Protocol), TS (Transport Stream),etc., and thus may be transmitted through various networks.

As described above, regarding the NAL unit, an NAL unit type may bespecified according to an RBSP data structure included in the NAL unit,and information regarding the NAL unit type stored in the NAL unitheader may be signaled.

For example, according to whether the NAL unit includes imageinformation (i.e., slice data), it may be roughly classified into a VCLNAL unit type and a non-VCL NAL unit. The VCL NAL unit type may beclassified according to a property, type, etc., of a picture included inthe VAL NAL unit, and the non-VCL NAL unit type may be classifiedaccording to a type, etc., of a parameter set.

The following is an example of an NAL unit type specified according tothe property and type of the picture included in the VCL NAL unit.

-   -   TSA (Temporal Sub-layer Access): A type for an NAL unit        including a coded slice segment of a TSA picture. Herein, the        TSA picture is a picture which can be switched between temporal        sub-layers in a bitstream supporting a temporal scalability, and        is a picture indicating a position at which up-switching is        possible from a lower sub-layer to an upper sub-layer.    -   STSA (Step-wise Temporal Sub-layer Access): A type for an NAL        unit including a coded slice segment of an STSA picture. Herein,        the STSA picture is a picture which can be switched between        temporal sub-layers in the bitstream supporting the temporal        scalability, and is a picture indicating a position at which        up-switching is possible from a lower sub-layer to a higher        sub-layer which is one level higher than the lower sub-layer.

TRAIL: A type for an NAL unit including a coded slice segment of anon-TSA, non-STSA trailing picture. Herein, the trailing picture impliesa picture which appears after a picture capable of random access interms of an output order and a decoding order.

IDR (Instantaneous Decoding Refresh): A type for an NAL unit including acoded slice segment of an IDR picture. Herein, the IDR picture is apicture capable of random access, and may be a first picture in abitstream in terms of a decoding order or may appear in the middle ofthe bitstream. In addition, the IDR picture includes only I slices. EachIDR picture is a first picture of a coded video sequence (CVS) in termsof the decoding order. If the IDR picture has a relation with adecodable leading picture to be described below, an NAL unit type of theIDR picture may be denoted by IDR_W_RADL, and if the IDR picture doesnot have the relation with the leading picture, the NAL unit type of theIDR picture may be denoted by IDR_N_LP. The IDR picture does not have arelation with an undecodable leading picture to be described below.

CRA (Clean Random Access): A type of an NAL unit including a coded slicesegment of a CRA picture. Herein, the CRA picture is a picture capableof random access, and may be a first picture in a bitstream in terms ofa decoding order or may appear in the middle of the bitstream. Inaddition, the CRA picture includes only I slices. The CRA picture mayhave a relation with a leading picture which can be decoded or of whichdecoding can be skipped. The leading picture of which decoding can beskipped may not be output. The leading picture of which decoding can beskipped may use a picture not existing in the bitstream as a referencepicture, and thus the leading picture of which decoding can be skippedby a decoder may not be output.

BLA (Broken Link Access): A type for an NAL unit including a coded slicesegment of a BLA picture. Herein, the BLA picture is a picture capableof random access, and may be a first picture in a bitstream in terms ofa decoding order or may appear in the middle of the bitstream. Inaddition, the BLA picture includes only I slices. In each BLA picture, anew coded video sequence (CVS) starts, and the same decoding process asthat used for the IDR picture may be performed. If the BLA picture has arelation with a leading picture of which decoding can be skipped, an NALunit type of the BLA picture may be denoted by BLA_W_LP, and if the BLApicture has a relation with a decodable leading picture, the NAL unittype of the BLA picture may be denoted by BLA_W_LP. If the BLA picturedoes not have a relation with the leading picture of which decoding canbe skipped but has a relation with the decodable leading picture, theNAL unit type of the BLA picture may be denoted by BLA_W_RADL. When theBLA picture does not have a relation with the leading picture, the NALunit type of the BLA picture may be denoted by BLA_N_LP.

The following is an example of an NAL unit type specified according tothe property and type of the picture included in the non-VCL NAL unit.

VPS (Video Parameter Set): A type for an NAL unit including a VPS.

SPS (Sequence Parameter Set): A type for an NAL unit including an SPS.

PPS (Picture Parameter Set): A type for an NAL unit including a PPS.

The aforementioned NAL unit types have syntax information for the NALunit type, and the syntax information stored in an NAL unit header maybe signaled. For example, the syntax information may be nal_unit_type,and the NAL unit types may be specified by a value ‘nal_unit_type’.

Meanwhile, a bitstream (or temporal scalable bitstream) supporting atemporal scalability includes information on a temporal layer which isscaled temporally. The information on the temporal layer may beidentification information of a temporal layer specified according to atemporal scalability of the NAL unit. For example, the identificationinformation of the temporal layer may use syntax informationtemporal_id, and the syntax information temporal_id may be stored in anNAL unit header in an encoding apparatus and may be signaled to adecoding apparatus. In the following description, the temporal layer mayalso be referred to as a sub-layer, a temporal sub-layer, a temporalscalable layer, etc.

FIG. 4 shows a temporal layer structure for NAL units in a bitstreamsupporting a temporal scalability.

If the bitstream supports the temporal scalability, NAL units includedin the bitstream have identification information (e.g., temporal_id) ofthe temporal layer. For example, a temporal layer consisting of NALunits of which temporal_id is 0 may provide a lowest temporalscalability, and a temporal layer consisting of NAL units of whichtemporal_id is 2 may provide a highest temporal scalability.

In FIG. 4, a box indicated by I is an I picture, and a box indicated byB is a B picture. In addition, an arrow mark indicates a referencerelation regarding whether one picture refers to another picture.

As shown in FIG. 4, the NAL units of a temporal layer of whichtemporal_id is 0 are reference pictures which can be referenced by NALunits of a temporal layer of which temporal_id is 0, 1, or 2. The NALunits of the temporal layer of which temporal_id is 1 are referencepictures which can be referenced by NAL units of a temporal layer ofwhich temporal_id is 1 or 2. NAL units of a temporal layer of whichtemporal_id is 2 may be reference pictures which can be referenced byNAL units of the same temporal layer, i.e., the temporal layer of whichtemporal_id is 2, or may be non-reference pictures which are notreferenced by a different picture.

As shown in FIG. 4, if NAL units of the temporal layer of whichtemporal_id is 2, that is, a highest temporal layer, are non-referencepictures, the NAL units may be extracted (or removed) from the bitstreamwithout having an effect on different pictures.

In order to facilitate a bitstream extraction process, the presentinvention may provide information indicating whether the NAL unit is areference picture or a non-reference picture. Such information may beprovided with an NAL unit level.

An NAL unit type according to an embodiment of the present invention maybe classified according to whether an NAL unit is a reference picturereferenced by a different picture or a non-reference picture notreferenced by the different picture.

For example, if a TSA picture is a reference picture, an NAL unit of theTSA picture may be denoted by TSA_R, and if the TSA picture is anon-reference picture, the NAL unit type of the TSA picture may bedenoted by TSA_N. If an STSA picture is a reference picture, an NAL unittype of the STSA picture may be denoted by STSA_R, and if the STSApicture is a non-reference picture, the NAL unit type of the STSApicture may be denoted by STSA_N. If a non-TSA, non-STSA trailingpicture is a reference picture, an NAL unit type of the non-TSA,non-STSA trailing picture may be denoted by TRAIL_R, and if the non-TSA,non-STSA trailing picture is a non-reference picture, the NAL unit typeof the non-TSA, non-STSA trailing picture may be denoted by TRAIL_N.

FIG. 5 shows a temporal layer structure for NAL units in a bitstreamsupporting a temporal scalability to which the present invention isapplicable.

If the bitstream supports the temporal scalability, NAL units includedin the bitstream have identification information (e.g., temporal_id) ofthe temporal layer.

For example, assuming a case where NAL units of which temporal_id is 0,1, 2 are included in the bitstream, as shown in FIG. 5, it may beclassified into a temporal layer 500 consisting of NAL units of whichtemporal_id is 0, a temporal layer 510 consisting of NAL units of whichtemporal_id is 1, and a temporal layer 520 consisting of NAL units ofwhich temporal_id is 2. In this case, the temporal layer 500 consistingof NAL units of which temporal_id is 0 may provide a lowest temporalscalability, and the temporal layer 520 consisting of NAL units of whichtemporal_id is 2 may provide a highest temporal scalability.

In FIG. 5, a box indicated by I is an I picture, and a box indicated byB is a B picture. In addition, an arrow mark indicates a referencerelation regarding whether one picture refers to another picture.

As shown in FIG. 5, the temporal layer 520 of which temporal_id is 2consists of pictures of a TRAIL_N type. As described above, the TRAIL_Ntype is information indicating an NAL unit of which a trailing pictureis a non-reference picture. The non-reference picture is not referencedby a different picture during an inter prediction, and thus can beremoved from the bitstream without having an effect on a decodingprocess of different pictures when extracting the bitstream. Therefore,pictures of the TRAIL_type of the temporal layer 520 of whichtemporal_id is 2 may not have an effect on the decoding even if thepictures are removed from the bitstream.

Meanwhile, among the aforementioned NAL unit types, IDR, CRA, and BLAtypes are information indicating an NAL unit including a picture capableof random access (or slicing), that is, a random access point (RAP) orintra random access point (IRAP) picture used as a random access point.In other words, the RAP picture may be an IDR, CRA, or BLA picture, andmay include only an I slice. According to a decoding order in thebitstream, a first picture is an RAP picture.

If the RAP picture (i.e., IDR, CRA, or BLA picture) is included in thebitstream, there may be a picture of which an output order is earlierthan that of the RAP picture whereas a decoding order is later than thatof the RAP picture. Such pictures are called leading pictures (LPs).

FIG. 6 is a diagram for explaining a randomly accessible picture.

The randomly accessible picture, that is, an RAP or IRAP picture used asa random access point, is a first picture in a bitstream in terms of adecoding order, and includes only an I slice.

An output order (or display order) and decoding order of the picture areshown in FIG. 6. As shown, the output order and decoding order of thepicture may differ from each other. For convenience of explanation, thepictures are divided by a specific group.

Pictures belonging to a first group I are pictures of which an outputorder and a decoding order are both earlier than those of IRAP pictures.Pictures belonging to a second group II are pictures of which an outputorder is earlier than that of the IRAP picture whereas a decoding orderis later than that of the IRAP picture. Pictures of a third group IIIare pictures of which an output order and a decoding order are bothlater than those of the IRAP picture.

The pictures of the first group I may be output by being decodedirrespective of the IRAP picture.

The pictures which belong to the second group II and which are outputearlier than the IRAP picture are called leading pictures. The leadingpictures may be problematic in a decoding process when the IRAP pictureis used as a random access point.

The pictures belonging to the third group III of which an output orderand a decoding order are later than those of the IRAP picture are callednormal pictures. The normal picture is not used as a reference pictureof the leading picture.

A random access point in which random access occurs in the bitstream isan IRAP picture, and the random access starts when a first picture ofthe second group II is output.

FIG. 7 is a diagram for explaining an IDR picture.

The IDR picture is a picture used as a random access point when a groupof picture has a closed structure. The IDR picture is an IRAP picture asdescribed above, and thus includes only an I slice. The IDR picture maybe a first picture in the bitstream in terms of a decoding order, andmay appear in the middle of the bitstream. When the IDR picture isdecoded, all reference pictures stored in a decoded picture buffer (DPB)are indicated by an “unused for reference”.

A bar shown in FIG. 7 indicates a picture, and an arrow mark indicates areference relation regarding whether the picture can use a differentpicture as a reference picture. A mark ‘x’ indicated on the arrow markindicates that a picture indicated by the arrow mark cannot bereferenced by a corresponding picture(s).

As shown, a picture of which a POC is 32 is an IDR picture. If the POCis 25 to 31, pictures which are output earlier than the IDR picture areleading pictures 710. Pictures of which a POC is greater than or equalto 33 correspond to a normal picture 720.

The leading pictures 710 of which an output order is earlier than thatof the IDR picture can use the IDR picture and a different leadingpicture as a reference picture, but cannot use a previous picture 730 ofwhich an output order and a decoding order are earlier than those of theleading pictures 710 as the reference picture.

The normal pictures 720 of which an output order and a decoding orderare later than those of the IDR picture may be decoded by referring tothe IDR picture, the leading picture, and a different normal picture.

FIG. 8 is a diagram for explaining a CRA picture.

The CRA picture is a picture used as a random access point when a groupof picture has an open structure. As described above, the CRA picture isalso an IRAP picture and thus includes only an I slice. The IDR picturemay be a first picture in the bitstream in terms of a decoding order,and may appear in the middle of the bitstream for a normal play.

A bar shown in FIG. 8 indicates a picture, and an arrow mark indicates areference relation regarding whether the picture can use a differentpicture as a reference picture. A mark ‘x’ indicated on the arrow markindicates that a picture indicated by the arrow mark cannot bereferenced by a corresponding picture or pictures.

Leading pictures 810 of which an output order is earlier than that ofthe CRA picture can use all pictures, i.e., a CRA picture, a differentleading picture, and previous pictures 830 of which an output order anda decoding order are earlier than those of the leading pictures 810 as areference picture.

On the other hand, normal pictures 820 of which an output order and adecoding order are later than those of the CRA picture may be decoded byreferring to the CRA picture and a different normal picture. The normalpictures 820 may not use the leading pictures 810 as the referencepicture.

A BLA picture implies a picture which has a similar function andproperty as the CRA picture and which exists in the middle of abitstream as a random access point when a coded picture is sliced or thebitstream is broken in the middle. However, since the BLA picture isregarded as a start of a new sequence at the occurrence of randomaccess, unlike the CRA picture, parameter information regarding an imagecan be entirely received again when the BLA picture is received by adecoder.

The BLA picture may be determined from an encoding apparatus, and theCRA picture may be changed to the BLA picture in a system which receivesthe bitstream from the encoding apparatus. For example, when thebitstream is sliced, the system changes the CRA picture into the BLApicture and thus provides it to the decoder which decodes an image. Inthis case, parameter information regarding the image is also newlyprovided from the system to the decoder. In the present invention, thedecoder implies a device including an image processor for decoding animage, and may be implemented with the decoding apparatus of FIG. 2, ormay imply a decoding module which is a core module for processing theimage.

As described above, the leading pictures are output earlier than the CRApicture according to the output order, but are decoded later than theCRA picture. At least one of previous pictures may be referenced by theleading pictures.

For example, when the bitstream is broken or lost in the middle or whenrandom access occurs in the CRA picture abruptly at the occurrence ofslicing of the picture, previous pictures of which a decoding order isearlier than that of the CRA picture may be unavailable. That is, sincethe previous pictures which may be used as a reference picture of theleading pictures are unavailable, the leading picture which refers tothe unavailable picture may not be normally decoded.

A case where the reference picture referenced by the leading picture isunavailable implies a case where the leading picture refers to a picturenot existing in the bitstream or a picture referenced by the leadingpicture does not exist in a decoded picture buffer (DPB) or is a picturemarked by an “unused for reference” in the DPB.

As described above, since the leading picture which refers to theunavailable reference picture may not be normally decoded, such aleading picture may be discarded in a decoding process. Therefore, atthe occurrence of the random access, information capable ofdistinguishing a decodable leading picture and an undecodable leadingpicture is required to a normal decoding process of the leading picture.Such information may be provided in an NAL unit type, and an embodimentof the NAL unit type for the leading picture is described below.

-   -   DLP_NUT: An NAL unit type (NUT) for an NAL unit including a        coded slice segment of a decodable leading picture (DLP). The        decodable leading picture implies a decodable leading picture        for random access. All decodable leading pictures for random        access are leading pictures. The decodable leading pictures for        random access are not used as a reference picture in a decoding        process of trailing pictures related to the same RAP (or IRAP)        picture. In the presence of the decodable leading pictures for        random access, the decodable leading pictures for random access        have a decoding order earlier than that of the trailing pictures        related to the same RAP (or IRAP) picture.    -   TFD_NUT: A type for an NAL unit including a coded slice segment        of a tagged for discard (TFD) leading picture which may be        discarded without being normally decoded when a picture which        appears earlier than the RAP picture is unavailable. The TFD        leading picture may be a skipped leading picture for random        access. The skipped leading pictures for random access are        leading pictures related to the BLA or CRA picture. The skipped        leading picture for random access can refer to pictures not        existing in the bitstream, and thus the skipped leading picture        for random access are not output and cannot be correctly        decoded. The skipped leading picture for random access are not        used as reference pictures in a decoding process of pictures        other than the skipped leading picture for random access. In the        presence of the skipped leading pictures for random access, the        skipped leading pictures for random access have a decoding order        earlier than that of the trailing pictures related to the same        RAP (or IRAP) picture.

The DLP and TFD leading picture may be processed in the same method asthat of the trailing pictures in the normal decoding process performedwhen random access or splicing does not occur.

FIG. 9 shows a temporal layer structure for NAL units including aleading picture in a bitstream supporting a temporal scalability.

If the bitstream supports the temporal scalability, NAL units includedin the bitstream have identification information (e.g., temporal_id) ofthe temporal layer.

For example, assuming a case where NAL units of which temporal_id is 0,1, 2 are included in the bitstream, as shown in FIG. 9, it may beclassified into a temporal layer 900 consisting of NAL units of whichtemporal_id is 0, a temporal layer 910 consisting of NAL units of whichtemporal_id is 1, and a temporal layer 920 consisting of NAL units ofwhich temporal_id is 2. In this case, the temporal layer 900 consistingof NAL units of which temporal_id is 0 may provide a lowest temporalscalability, and the temporal layer 920 consisting of NAL units of whichtemporal_id is 2 may provide a highest temporal scalability.

In FIG. 9, a box indicated by I is an I picture, and a box indicated byB is a B picture. In addition, an arrow mark indicates a referencerelation regarding whether one picture refers to another picture. Forexample, a TRAIL_R picture of the temporal layer 910 of whichtemporal-id is 1 uses a IDR_N_LP picture and a TRAIL_R picture of thetemporal layer 900 of which temporal_id is 0 as a reference picture, andis used as a reference picture by TRAIL_N pictures of the temporal layer920 of which temporal_id is 2.

In the example of FIG. 9, an NAL unit type can be used to know thatTRAIL_N pictures of the temporal layer 920 of which temporal_id is 2 arepictures not referenced by different pictures. In this case, similarlyto the embodiment of FIG. 5 described above, the TRAIL_N pictures notreferenced when the bitstream is extracted may be removed from thebitstream. However, DLP_NUT and TFD_NUT leading pictures of the temporallayer 920 of which temporal_id is 2 do not include information capableof distinguishing whether the pictures are pictures referenced bydifferent pictures. Therefore, it is difficult to determine whether ithas an effect on a decoding process even if the leading picture isremoved from the bitstream in the bitstream extraction process.

In order to solve the aforementioned problem, the present inventionprovides information indicating whether the leading picture is a picturereferenced by a different picture. According to an embodiment of thepresent invention, an NAL unit type for the leading picture is definedas follows.

-   -   DLP_R: A referenced decodable leading picture. In other words, a        type for an NAL unit including a coded slice segment of a random        access decodable leading (RADL) referenced by a different        picture.    -   DLP_N: A non-referenced decodable leading picture. In other        words, a type for an NAL unit including a coded slice segment of        a random access decodable leading (RADL) picture not referenced        by a different picture.    -   TFD_R: A referenced undecodable picture (i.e., a referenced TFD        picture). In other words, a type for an NAL unit including a        coded slice segment of a leading picture which may not be        normally decoded when a picture which appears earlier than the        RAP picture is unavailable and which is referenced by a        different picture. The TFD leading picture is a picture which        may be discarded (or skipped), and may be called a random access        skipped leading (RASL) picture.    -   TFD_N: A non-referenced undecodable picture (i.e.,        non-referenced TFD picture). In other words, a type for an NAL        unit including a coded slice segment of a leading picture which        may not be normally decoded when a picture which appears earlier        than the RAP picture is unavailable and which is not referenced        by a different picture. The TFD leading picture is a picture        which may be discarded (or skipped), and may be called a random        access skipped leading (RASL) picture.

The aforementioned NAL unit types DLP_R, DLP_N, TFD_R, TFD_N for theleading picture according to the embodiment of the present invention maybe defined by using reserved NAL unit types which are not yet used by adifferent type. In addition, the NAL unit types DLP_R, DLP_N, TFD_R,TFD_N for the leading picture according to the embodiment of the presentinvention may be signaled by being stored in syntax information (e.g.,nal_unit_type) for the NAL unit type of an NAL unit header.

The followings are examples applicable to NAL unit types DLP_R, DLP_N,TFD_R, TFD_N for a leading picture according to an embodiment of thepresent invention.

-   -   When the NAL unit type (e.g., nal_unit_type) is TFD_N or DLP_N,        a picture to be decoded is not included in a reference picture        set (RPS) of a picture having identification information (e.g.,        temporal_id) of the same temporal layer.

The RPS implies a set of reference pictures of a current picture, andmay consist of reference pictures of which a decoding order is earlierthan that of the current picture. The reference picture may be used inan inter prediction of the current picture. Herein, the RPS may be ashort term reference picture set (e.g., RefPicSetStCurrBefore,RefPicSetStCurrAfter) and a long term reference picture set (e.g.,RefPicSetLtCurr) which consist of reference pictures of which a pictureorder count (POC) order is earlier than or later than that of thecurrent picture.

-   -   A coded picture of which an NAL unit type (e.g., nal_unit_type)        is TFD_N or DLP_N may be discarded without having an effect on a        decoding process of different pictures having identification        information (e.g., temporal_id) of the same temporal layer. This        is because whether the coded picture with TFD_N or DLP_N is a        picture referenced by different pictures can be known by using        the NAL unit type, and can be extracted from a bitstream since        it is not used as a reference picture in decoding.    -   A coded picture of which an NAL unit type (e.g., nal_unit_type)        is TFD_N or DLP_N may be processed similarly to the        aforementioned TRAIL_N, TSA_N, or STSA_N picture if not a case        where decoding starts from a random access point related to a        leading picture.    -   A coded picture of which an NAL unit type (e.g., nal_unit_type)        is TFD_R or DLP_R may be processed similarly to the        aforementioned TRAIL_R, TSA_R, or STSA_R picture if not a case        where decoding starts from a random access point related to a        leading picture.

FIG. 10 is a diagram for explaining an operation of removing NAL unitsincluding a leading picture from a bitstream according to an embodimentof the present invention.

If the bitstream supports the temporal scalability, NAL units includedin the bitstream have identification information (e.g., temporal_id) ofthe temporal layer.

For example, assuming a case where NAL units of which temporal_id is 0,1, 2 are included in the bitstream, as shown in FIG. 10, it may beclassified into a temporal layer 1000 consisting of NAL units of whichtemporal_id is 0, a temporal layer 1010 consisting of NAL units of whichtemporal_id is 1, and a temporal layer 1020 consisting of NAL units ofwhich temporal_id is 2. In this case, the temporal layer 1000 consistingof NAL units of which temporal_id is 0 may provide a lowest temporalscalability, and the temporal layer 1020 consisting of NAL units ofwhich temporal_id is 2 may provide a highest temporal scalability.

In FIG. 10, a box indicated by I is an I picture, and a box indicated byB is a B picture. In addition, an arrow mark indicates a referencerelation regarding whether one picture refers to another picture.

As shown in FIG. 10, the temporal layer 1020 of which temporal_id is 2consists of TRAIL_N pictures and TFD_N and DLP_N leading pictures. Asdescribed above, since the TRAIL_N picture is a trailing picture notreferenced by a different picture, it can be removed from a bitstreamwithout having an effect on a decoding process of different pictures.

In addition, since the leading picture is signaled by being defined withan NAL unit type indicating whether it is a picture referenced by adifferent picture according to the embodiment of the present invention,whether the leading picture can be removed from the bitstream can beknown by using the NAL unit type. As shown in FIG. 10, the TFD_N andDLP_N leading pictures are leading pictures not referenced by adifferent picture, and can be removed from the bitstream without havingan effect on a decoding process of the different pictures. That is,since information regarding whether the leading picture is a referencepicture or a non-reference picture can be derived from the NAL unittype, a bitstream extraction process for the leading picture may beperformed similarly to a bitstream extraction process of a trailingpicture. Therefore, since pictures corresponding to the temporal layer1020 of which temporal_id is 2 are non-reference pictures, thenon-reference pictures of the temporal layer 1020 of which temporal_idis 2 may be extracted from the bitstream in decoding.

FIG. 11 is a flowchart showing an encoding method of image informationaccording to an embodiment of the present invention. The method of FIG.11 may be performed in the aforementioned encoding apparatus of FIG. 1.

Referring to FIG. 11, the encoding apparatus determines an NAL unit typeaccording to whether an NAL unit is a reference picture (step S1100). Inthis case, the NAL unit may be an NAL unit including a residual signalfor a current picture generated by performing an inter prediction on thebasis of the current picture.

The encoding apparatus may determine the NAL unit type according toinformation (i.e., the residual signal for the current picture) includedin the NAL unit. For example, the NAL unit type may be determinedaccording to whether the NAL unit is a leading picture referenced by adifferent picture or a leading picture not referenced by the differentpicture. The leading picture implies a picture of which an output orderis earlier than that of a random access point picture and an decodingorder is later than that, and may include a first leading picture whichcan be decoded and a second leading picture which cannot be decoded.

If the NAL unit is a first leading picture referenced by a differentpicture, the encoding apparatus may determine the NAL unit type to DLP_Ror RADL_R. If the NAL unit is a first leading picture not referenced bythe different picture, the encoding apparatus may determine the NAL unitto DLP_N or RADL_N.

Otherwise, if the NAL unit is a second leading picture referenced by thedifferent picture, the encoding apparatus may determine the NAL unittype to TFD_R or RASL_R, and if the NAL unit is a second leading picturenot referenced by the different picture, the encoding apparatus maydetermine the NAL unit type to TFD_N or RASL_N.

The encoding apparatus encodes and transmits a bitstream includinginformation on the NAL unit and the NAL unit type (step S1110).

The encoding apparatus may encode the information on the NAL unit typeby using a nal_unit_type syntax and may store it in an NAL unit header.In addition, in case of a bitstream supporting a temporal scalability,the encoding apparatus may generate a bitstream further includingidentification information of a temporal layer for identifying atemporal scalable layer of an NAL unit. The identification informationof the temporal layer may be encoded with the temporal_id syntax and maybe stored in the NAL unit header.

FIG. 12 is a flowchart showing a decoding method of image informationaccording to an embodiment of the present invention. The method of FIG.12 may be performed in the aforementioned decoding apparatus of FIG. 2.

Referring to FIG. 12, the decoding apparatus receives a bitstreamincluding information on an NAL unit (step S1200).

The information on the NAL unit includes information on an NAL unit typedetermined by a property and type of a picture included in the NAL unit.As to the NAL unit type, in addition to a property and type of thepicture included in the NAL unit, information regarding whether thepicture included in the NAL unit is a reference picture may also bederived together.

For example, the information on the NAL unit type may be included in thebitstream and thus be stored in an NAL unit header by usingnal_unit_type syntax. Since the NAL unit type has been described abovein detail, an explanation thereof will be omitted herein.

In addition, the information on the NAL unit may further includeidentification information of a temporal layer supporting a temporalscalability. The identification information of the temporal layer may belayer identification information for identifying a temporal scalablelayer of a corresponding NAL unit. For example, the identificationinformation of the temporal layer may be included in the bitstream andthus be stored in an NAL unit header by using the temporal_id syntax.

The decoding apparatus decodes the NAL unit by confirming whether theNAL unit in the bitstream is a reference picture on the basis of theinformation on the NAL unit type (step S1210).

The information on the NAL unit type may be used to derive whether theNAL unit is a reference picture referenced by a different picture or anon-reference picture not referenced by the different picture. If theNAL unit is the non-reference picture not referenced by the differentpicture, the NAL unit may be removed by extracting from the bitstream ina decoding process.

For example, the information on the NAL unit type may be informationindicating whether the NAL unit is a leading picture referenced by thedifferent picture or whether the NAL unit is a leading picture notreferenced by the different picture. The leading picture implies apicture of which an output order is earlier than that of a random accesspoint picture and an decoding order is later than that, and may includea first leading picture which can be decoded and a second leadingpicture which cannot be decoded.

If the NAL unit type included in the bitstream is DLP_R or RADL_R, thedecoding apparatus may know that the NAL unit is a first leading picturereferenced by the different picture. If the NAL unit type included inthe bitstream is DLP_N or RADL_N, the decoding apparatus may know thatthe NAL unit is a first leading picture not referenced by the differentpicture.

Otherwise, if the NAL unit type included in the bitstream is TFD_R orRASL_R, the decoding apparatus may know that the NAL unit is a secondleading picture referenced by the different picture. If the NAL unittype included in the bitstream is TFD_N or RASL_N, the decodingapparatus may know that the NAL unit is a second leading picture notreferenced by the different picture.

In this case, if the NAL unit type is DLP_N or RADL_N, TFD_N or RASL_N,the decoding apparatus may extract the NAL unit corresponding to the NALunit type from the bitstream and then may perform decoding.

In addition, the decoding apparatus may derive a temporal layer of theNAL unit by using identification information of the temporal layer. IfNAL units of the same temporal layer are pictures (e.g., DLP_N or RADL_Npicture, TFD_N or RASL_N picture) not referenced by a different picture,the NAL units of the temporal layer may be removed from the bitstream.Herein, the NAL units of the same temporal layer imply NAL units havingthe same identification value of the temporal layer.

Although the aforementioned exemplary system has been described on thebasis of a flowchart in which steps or blocks are listed in sequence,the steps of the present invention are not limited to a certain order.Therefore, a certain step may be performed in a different step or in adifferent order or concurrently with respect to that described above.The aforementioned embodiments include various exemplary aspects.Therefore, all replacements, modifications and changes should fallwithin the spirit and scope of the claims of the present invention.

The invention claimed is:
 1. An image decoding method, the methodcomprising: receiving, by a decoding apparatus, a bitstream; acquiring,by the decoding apparatus, information indicating a network abstractionlayer (NAL) unit type from the bitstream; determining, by the decodingapparatus, a NAL unit type of a leading picture as one of NAL unittypes, based on the information indicating the NAL unit type, whereinthe leading picture precedes an associated random access point picturein output order; configuring, by the decoding apparatus, a referencepicture list to perform inter prediction with regard to a picture whichfollows the leading picture in decoding order, based on the NAL unittype of the leading picture; and performing, by the decoding apparatus,the inter prediction on at least one block in the picture based on thereference picture list, wherein the NAL unit types includes a first NALunit type representing referenced decodable leading picture and a secondNAL unit type representing non-referenced decodable leading picture. 2.The method of claim 1, wherein the NAL unit types further includes athird NAL unit type representing referenced skipped leading picture, anda fourth NAL unit type representing non-reference skipped leadingpicture, and wherein if the NAL unit type of the leading picture is thesecond NAL unit type or the fourth NAL unit type, the leading picture isnot configured in the reference picture list.
 3. The method of claim 1,wherein the NAL unit types further includes a third NAL unit typerepresenting referenced skipped leading picture, and a fourth NAL unittype representing non-reference skipped leading picture, and wherein ifthe NAL unit type of the leading picture is the second NAL unit type orthe fourth NAL unit type, the leading picture cannot be used for interprediction of subsequent pictures of the same temporal layer in thedecoding order.
 4. The method of claim 3, wherein in the configuring ofthe reference picture list, the leading picture of which an NAL unittype is the first NAL unit type and the third NAL unit type is includedin the reference picture list, and the leading picture is a picturewhich follows the random access point picture in the decoding order andprecedes the random access point picture in the output order.
 5. Themethod of claim 3, wherein if the NAL unit type of the leading is thesecond NAL unit type or the fourth NAL unit type, the leading picturecan be used for inter prediction of subsequent pictures of highertemporal layers.
 6. The method of claim 1, wherein the random accesspoint picture in which random access occurs is an instantaneous decodingrefresh (IDR) picture, and wherein the NAL unit type of the leadingpicture is the first NAL unit type of the second NAL unit type.
 7. Themethod of claim 2, wherein the random access point picture in whichrandom access occurs is a clean random access (CRA) picture or a brokenlink access (BLA) picture, and wherein if the NAL unit type of theleading picture is the second NAL unit type and the fourth NAL unittype, the leading picture is not output.
 8. The method of claim 1,wherein the random access point picture in which random access occurs isan instantaneous decoding refresh (IDR) picture, a clean random access(CRA) picture, or a broken link access (BLA) picture, and whereinpictures stored in a memory are marked with a non-referenced picture. 9.The method of claim 2, wherein if random access occurs in the randomaccess point picture and the NAL unit type of the leading picture is thesecond NAL unit type and the fourth NAL unit type, the leading pictureis removable without having an effect on the decoding of other picturesof the same temporal layer.